Electroplating tin



Patented Jan. 8, 1946 William B. Cooper and more, Md, assignors Company,Inc., Baltimore,

of New York Charles E. Glock, Bamto Crown Cork & Seal Md., a corporationNo Drawing. Application July 11, 1942, Serial No. 450,610

1 Claim. (01. 204-54) This invention relates to electroplating with tinand particularly to a bath for plating an indefinite length of blackiron or steel strip whereby the same may be continuously passed throughthe plating bath and provided with a smooth uniform matte 'or satinfinish.

The primary object of the invention is to pro vide an alkaline tinplating bath containing an alkali metal stannate, such as sodiumstannate. and which will be stable over a wide range of operatingconditions.

' tions, due to the use of ioopers so Another object of the invention isto provide a bath in which the sodium stannate is maintained withinsubstantially constant critical limits and 1 without requiring thatstannate be added during plating. I

A further object of the invention is to provide a sodium stannate bathin which a balance is constantly maintained between the amount of tindissolved from the anodes and the amount deposited on the travelingstrip by means of a critical alkali content whereby the tin content ofthe solution does not change and addition of alkali stannate duringcoating is made unnecessary.

An additional object of the invention is to provide a bath containing asoapy agent which will produce a foam on the surface of the plating so-.

lution, and prevent the solution from"spraying, due to hydrogen beingliberated at the cathode and oxygen at the anode.

0i! equal importance with the foregoing, it is an object of theinvention to provide an alkaline tin plating bath containing sodiumstannate in which the ratio of current density of the anode to thecathode may be maintained substantially equal, 1. e., 121, and with ananode current density of substantially 35 amperes per square foot,without requiring stannate additions during plating.

Another object of the invention is to provide an alkaline sodiumstannate bath in which it is unnecessary to maintain a greenish-yellowfilm in order to produce smooth and uniform electrodeposited coatings oftin.

Another object of the invention is to provide a bath in which anoxidizing agent, such as hydrogen peroxide, need not be employed,

Incarrying out the present invention, we will describe the same inconnection with the electrotin plating of 18 inch to 32 inch width steelstrip 10! indefinite length. The strip is continuously traveled, firstthrough an acid dip, then through a water rinse, then through theelectroplating bath, after which it is spray-washed'and dried.

The operations are carried out in a "line, and the strip never stopsduring the various operathe ends of the coils of strip can'be weldedtogether and the finished coated coils stripped while the band is incontinuous motion.

By strip steel, we mean various grades available and useful for themanufacture of contain ers and container closures, among which maybementioned low carbon rimmed steel, low carbon killed steel, andrephosphorized steel, as well as ordinary black iron strip. The strip isof the usual thicknesses employed for making containers and closure capsof sheet metal.

The pickling operation on the traveling strip is preferably carried outin a sulfuric acid solution of 3 to'6% H2304 by weight in water at 170F., using an inhibitor to prohibit etching of the strip. The sheet as itleaves the acid tank is'covered by a mm of acid since otherwise, asuperficial oxide will form on its surface and the sub- The travelingstrip is then rinsed free of acid in a simple dip tank having cleanwater constantly replacing the acid contaminated water resulting fromthe carry over of acid. The tank is bailled so that fresh water entersas the sheet leaves and is drained ofi at the entry side. The rinsewater is removed from the sheet as close to the plating bath as ispractical. This is done to eliminate the superficial oxidation that willresult whenever the sheet is exposed for a short time to the atmosphere.The faster the strip travels, the less trouble is experienced with thistype of oxidation. A squeegee roll is employed to remove most or therinse water and so constructed that a residual, continuous film of waterremains on the sheet whereby the strip is protected until immersion inthe plating solution to-which it now passed.

A preferred plating'solution found to give consistently good results hassubstantially the following formula:

Tin (as sodium stannate) Substantially 30 g./l. Sodium hydroxideSubstantially 10/12 g./l.

Hydrogen peroxide (optional)? Substantially pt. every A hr. to 10,500gallon tank. Substantially Ridosol #14 suflicient amount to obtain ablanket of foam. 1 qt. to 10,500 allons of plating solut on. TemperatureAbout F.

The plating tank has a capacity of about.10,500 gallons, and the tinanodes are disposed vertically therein in spaced relation. The travelingsituated that j strip is coated while traveling parallel to the anodesurfaces in the above bath.

When the caustic content is allowed to go as low as 9 g./l. (as NaOH),excessive sludging of the bath results, and if raised above 16 g./l., agood smooth coating cannot be obtained. When the tin content drops below30 g./l., the plating becomes very dull and loses its satin or matteappearance. As the tin content goes above 45 g./l.,

' tin tends to deposit on the current carrying rolls,

forming objectionable tin pimples." These put small dents in the stripand must be scraped from the rolls.

We have discovered that if the caustic content is kept within thecritical limits described, a balance is maintained between the amount oftin dissolved from the anodes and the amount deposited on the traveling.strip. In thi manner, the tin content of the solution does not change,as demonstrated by the factthat n additions of sodium stannate werenecessary over a severalmonth period with a line operating at speeds upto 370 ft./-min. on 29" wide material 24 hours a day and seven days aweek. An anode current density of 35 amp/sq. ft. is used in normaloperation, and the anode to cathode ratio is roughly 1:1. This currentdensity allows the anode to corrode uniformly, and no special treatmentis necessary to maintain the anodes in good condition.

"RidosoP #14 is an agent which is of soapy character and is added to thealkaline tin plating bath to produce a foam on the surface of theplating solution. This foam prevents the solution from spraying, whichis caused by hydrogen being liberated at the cathode and oxy en at theanode. These gases, rising out of the solution as very fine bubbles,carry some of the caustic plating solution with them and thereby causesdiscomfort to the people workingin the vicinity. In addition, parts ofthe nearby machinery are coated with this caustic compound. 'I'he'foarricollects these gases and subsequently releases them in larger masses andat a slower velocity.

The RidosoY' alsoreduces the surface tension of the solution, thusincreasing its detergent properties. The solution has a greater tendencyto plate more uniformly even on fairly greasy surfaces.

If desired, sodium acetate can be used in the bath but is not necessaryafter the bath has been in operation for a time. Its primary actionseems to be in builering the solution.

In order to eliminate the use of hydrogen peroxide, we employ one ironanode 15" wide by St" long, which appears to guard against formation ofspongy deposits.

ing as by a fusing operation, the brightening appears to be facilitated.Sodium cyanide is not preferred due to its creation of a health hazard.Sodium orthosilicate in amount of about up to 13 grams per liter may beused to replace caustic coda with good results when the bath is fresh,but on aging for a few days, a silicious precipitats forms. Thisprecipitate canbe redissolved by additions of caustic soda. This bath isnot considered to be as practical in operation as one using caustic. Byway of example, the composition of an orthosilicate bath is:

Tin (as sodium stannate) g./l. 30 Sodium orthosilicate g./l-- 13 Sodiumacetate g'./i it Sodium phosphate to replace sodium hydroxide may beadded to the stannate bath in amount of up to about 25 g./l.' and a goodplating obtained.

Also, an alkaline'bath made up of sodi suiflde replacing sodiumhydroxide and sodium stan= mate as the principal constituents willproduce satisfactory tin plating. An example of the com position of asodium sulfide bath is:

Tin (as sodium stannate) -g./i 5t Sodium sulfide (Na-2S) g./i it Thesolution was filtered and operated at 1T0 F. giving very good results.

Sodium orthosilicate, sodium phosphate and sodium sulfide have all beenused to replace sodium hydroxide, all or in part.

In the use of the alkali, such as the caustic or the equivalent alkalinematerials, the amount present in the bath is always efiective to maintain a constant balanc between the quantities of anode dissolved tin anddeposited tin, whereby plating on a strip of indefinite length may becarried out continuously without necessity of adding stannateto the bathduring the coating.

The strip leaves the plating tank through two sets of wringer rolls. Thefirst set removes most of the plating solution. The second set issupplied with water sprays which wash most of the remainder of thesolution back into the plating tank. A balance is maintained between therinse water entering the tank and water lost by evaporation so thatthe'amount of solution is con stant. The solution is constantly filteredto re move any coarserparticles.

The coated strip, after leaving the plating bath. is subjected to highpressure hot water sprays in a rinse tank. These sprays thoroughly cleanthe strip without scratching or otherwise changing the surface of thecoating. It then passes through wringer rolls to a drying tunnelconsisting of a blast of hot air and then the strip is continuouslycoiled and is ready for the next operation. This step can consist ofshearing to sheets or a reflowing or brightening operation by fusing thetin surface or cold rolling. v

A tabulation of line speed and coating weights is here shown forillustrating the advantageous results obtained by employing the criticalranges of caustic andstannate specified:

[Speed vs. coating on 29" wide (average) strip] Solution composition TiFtJmiu. Am e... mtg;

G. Sn/l G. NaOH/l LbJbb. 40, 000 42. 41 i0. 51 32 40,000 36. 94 10.03 4040, 000 36. 94 l0. l2 50 40, 000- 39. 52 i1. 11 56 40, 000 39. 27 ll. 3540, 000 40. 22 10. 95 40, 000 38. 70 10. 03 81 While we have referredherein to continuous strip metal as being successfully coated, it is tobe understood that other continuous materials, such as wire, as well asindividual objects, are

susceptible of being electroplated with equally satisfactory results.

Rodosol 14 is a product the composition of which usual y is of complexorganic nature, that is, sulphonated hydrocarbons. It is oi. a seaforming material and can be replaced by soaps such as sodium palmitate,oleate, etc.

We claim:

A process for the electrodeposition 0! metal- I .10 anode to cathoderatio of about 1:1.

lic tin comprising conducting an electrolizing current from a tin anodeto the article to be coated as a cathode through an aqueous solutionhaving a temperature oi about 190 F. and comprising essentially sodiumstannate to give substan- 1 tially 30 to 45 grams per liter of tin andsodium hydroxide substantially 10 to 12 grams per liter 'to produce asubstantially constant balancebe-f tween the quantities of anodedissolved tin and deposited tin, maintaining an anode current density ofsubstantially thirty five amperes per square foot with a ratio ofcurrent densities'ot anode to cathode or substantially 1:1 andan wnmm a.000mm. CHARLES E. cnocx

